Method for hydrogen production

ABSTRACT

The present invention relates to a method for hydrogen production and to a method of hydrogen and/or carbon dioxide production from syngas. The method comprises the steps of: (i) providing a gas stream comprising hydrogen and carbon monoxide, (ii) separating at least part of hydrogen from the stream yielding a hydrogen-depleted stream, (iii) subjecting the hydrogen-depleted stream to a water-gas shift reaction, and (iv) separating hydrogen from the stream resulting from step (iii). The method according to the invention improves the conversion of carbon monoxide in the water gas shift reaction and allows to increase the hydrogen production by 10-15% and to increase the overall energy efficiency of the system by 5-7%. The invention further relates to a plant for hydrogen and/or carbon dioxide production suitable for the method of the invention.

FIELD OF THE INVENTION

The invention relates to the field of hydrogen production fromhydrocarbons and, preferably, by steam reforming, autothermal reformingor catalytic partial oxidation of hydrocarbons.

BACKGROUND OF THE INVENTION

Hydrogen production from fossil fuels and especially by steam reformingof methane or other hydrocarbons is currently the most common processfor the production of hydrogen on an industrial scale. In this process,fossil fuel, for example natural gas or methane, is reacted with steamat high temperatures (700-1100° C., typically 700-900° C.) to producesynthesis gas (syngas), a gas mixture primarily made up of hydrogen (H₂)and carbon monoxide (CO). Syngas can also be obtained by autothermalreforming or by catalytic partial oxidation of hydrocarbons. Further,the syngas can be reacted with steam at a lower temperature in a watergas shift (WGS) reaction, to form carbon dioxide (CO₂) and hydrogen. Inthis way the hydrogen recovery from the hydrocarbon feed is furtherincreased.

Since in the above reactions CO and CO₂ are formed, production ofhydrogen is associated with the emission of significant amounts ofcarbon oxides, which are considered greenhouse gases. In fact, duringhydrogen production by natural gas reforming, more CO₂ is emitted thanH₂ produced. In addition, since the reforming reaction employs hightemperatures, a considerable amount of fuel is needed to be burnt tomaintain the required temperatures, which further contributes to thehigh CO₂ emission.

The carbon dioxide emission and the fuel needed for the combustion canbe decreased if the efficiency of the steam reforming process isimproved. Currently, the thermal efficiency achieved at existinghydrogen plants is only 65-75% and, therefore, efficiency improvement isdesired.

Methods are known in the prior art to improve the efficiency of thesteam reforming processes. For example, US 2008/000035 A1 describes amethod for hydrogen production wherein the water gas shift reaction isperformed in an integrated water gas shift/hydrogen separation membranesystem. In this system, hydrogen is separated in the same reactor wherethe water gas shift reaction occurs, which improves the processefficiency.

G. Barigozzi et al., Int Journal of Hydrogen energy, 36 (2011),5311-5320 discloses several configurations wherein a membrane separationunit is placed before and after the water gas shift reactor or beforethe PSA unit. Barigozzi concludes that a configuration with the membraneunit placed after water gas shift reactor leads to the most efficientoverall process.

Although attempts have been made to increase hydrogen production andefficiency in steam reforming, it is still desirable to further improvethe overall efficiency of the process and the hydrogen recovery from ahydrocarbons-containing feed. In addition, it is desired to producehydrogen having a lower caloric value of the feed and fuel needed toproduce a volume of hydrogen (kcal/Nm³). Hydrogen produced with a lowcaloric value of the feed is associated with lower production costs anda lower emission of carbon oxides caused by the production.

SUMMARY OF THE INVENTION

In order to better address one or more of the foregoing desires, theinvention, in one aspect, provides a method for the production ofhydrogen comprising the steps of:

(i) providing a gas stream comprising hydrogen and carbon monoxide;

(ii) separating at least 70 vol. %, preferably at least 80 vol. % ofhydrogen from the stream by membrane separation using a thinpalladium-based membrane having a thickness of 1-3 μm, yielding ahydrogen-depleted stream;

(iii) subjecting the hydrogen-depleted stream to a water-gas shiftreaction; and optionally

(iv) separating hydrogen from the stream resulting from step (iii).

In another aspect, the invention relates to a method of hydrogen andcarbon dioxide production, wherein additionally to the foregoing stepscarbon dioxide is separated from the stream resulting from step (iii).

In yet a further aspect, the invention relates to a plant for hydrogenproduction, suitable for the method according to the invention,comprising a reactor 110 for the production of syngas from hydrocarbons,wherein an outlet of the reactor 110 is connected to an inlet of a firstseparation module 101, 101A for hydrogen separation having an outlet forhydrogen and an outlet for a hydrogen-depleted stream, wherein theoutlet for the hydrogen-depleted stream is connected to an inlet of ashift reactor 103 for water gas shift reaction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical process scheme for hydrogen production known inthe prior art.

FIG. 2 shows a process scheme for hydrogen production according to apreferred embodiment of the invention.

FIG. 3 shows a process scheme for hydrogen and carbon dioxideproduction, according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention, in a broad sense, is based on the judicious insight toperform an additional hydrogen separation step in a typical hydrogenproduction system, before the water gas shift reaction, and to performthis using a thin membrane having a thickness of 1 μm to 3 μm. Thisadditional separation step allows to increase hydrogen production and todecrease the caloric value of the produced hydrogen. Also, the overallefficiency of the production process is increased. In the invention,thus a judicious combination is provided of thin membranes, andpositioning of these membranes upstream of a water gas shift reactor. Ascompared to positioning of membranes for hydrogen separation after thewater gas shift reaction, the invention provides for a process enjoyingsignificant advantages. Particularly, a high flux is obtained, andallowing a hydrogen depleted stream to enter the water gas shiftreactor, provides for a more economical and efficient process.

The use of the aforementioned thin membranes provides an advantage inthat it opens up a possibility to further enhance the trans-membraneflux of hydrogen. This can be executed by applying a sweep gasdownstream of the membrane. The sweep gas should be substantially inertto hydrogen. Without detracting from other inert gases or gas mixturesthat can be used, such as noble gases, or steam, the preferred gas isnitrogen. The sweep gas, which serves to remove hydrogen, in effectleads to a decrease of the hydrogen partial pressure at the downstreamside of the membrane. By virtue of the relatively low membrane thicknessof 1 μm to μm, the upstream gas flow of hydrogen will be directlyaffected by the resulting steeper trans-membrane gradient of hydrogenpressure. This, in effect, causes an increased trans-membrane flux ofhydrogen to occur.

The process of the invention starts with making or obtaining a hydrogenrich gas, or preferably, synthesis gas (syngas). Syngas is a gas mixturecontaining hydrogen and carbon monoxide and is usually obtained fromhydrocarbons. Any suitable source of syngas can be used in the presentinvention. It is however preferred that the syngas is obtained by steamreforming (SR), by autothermal reforming (ATR) or by catalytic partialoxidation (CPO) processes. More preferably, the source of the syngas isa steam reforming reaction, such as obtained downstream to a steamreformer.

The important step of the present invention is the separation of atleast 70%, preferably at least 80% of hydrogen from the syngas beforethe water gas shift reaction. In case the syngas is obtained from asteam reformer, the method according to the invention is in fact amodification of an existing steam reforming process employing the stepsof steam reforming and a WGS reaction, wherein hydrogen is additionallyseparated from the syngas stream before the WGS reactor. The hydrogenseparation is preferably done in a separate device and more preferably,in a membrane separator. Hydrogen selective membranes are known to askilled person and are commercially available. Preferably, hydrogenpermeable membranes characterised by high hydrogen selectivity and highH₂ flow are used, such as palladium-based membranes. In the inventionthin palladium membranes are used, generally having a thickness of from1 to 3 μm. These membranes can be manufactured according to the methoddescribed in “Membrane Reactors for hydrogen production processes”, M.De Falco, L. Marrelli, G. Iaquaniello (Eds.), Springer, 2011.

If needed, the syngas as obtained from a SR, ATR or CPO reactor iscooled down to a working temperature of the separation membrane used,before entering the separation module, in order to avoid damaging themembrane. The cooling can, for example, be done in a process gas boiler,or by direct quenching. It is thus preferred to cool the gas stream to atemperature lower than the maximum working temperature of the hydrogenseparator and even more preferred to at least 5° C. lower. In case ofthe above mentioned palladium-based membranes, the working temperatureis typically in the range of 350-500° C., or preferably in the range400-450° C. Therefore, the syngas is preferably cooled to a temperatureof at most 450° C. before entering the membrane separation reactor, andpreferably to at most 445° C.

As mentioned above, at least 70 vol. % of the hydrogen contained in thesyngas stream is separated. The hydrogen separation at this stageimproves the subsequent conversion of carbon monoxide in the WGSreaction and therefore the general process efficiency. Preferably, atleast 75 vol. % and more preferably at least 80 vol. % of the hydrogenis separated. Although all hydrogen can be separated, the bestimprovement of the overall efficiency and hydrogen production isachieved when between 70 and 90 vol. % of total hydrogen present in thestream, more preferably between 80 and 90 vol. %, is separated.

Pure hydrogen that is separated from the syngas stream is at lowpressure and can further be compressed and delivered at battery limit.It can also be combined with other hydrogen-rich streams, and/or it canbe additionally purified by pressure swing absorption (PSA) to obtainultra pure hydrogen.

The hydrogen-depleted stream after the hydrogen separation step isfurther routed to a water gas shift reactor (WGSR), wherein the carbonmonoxide and steam are converted to hydrogen and carbon dioxide. In apreferred embodiment, the temperature of the hydrogen-depleted stream isadapted to the temperature of the WGSR, before entering the reactor,which can be done in a heat exchanger. Preferably, the stream is cooledto the inlet temperature of the WGS reactor.

The WGS reaction is typically carried out using either a single stage ormulti stage process to attain the desired degree and rate of conversion.In a multi stage process, the high temperature stage (HTS) operates at300-450° C. and typically in the presence of an iron-based catalyst suchas Fe/Cr. In the HTS the largest amount of CO is converted. In thefollowing stage, medium or low temperature stage (MTS or LTS), theoperating temperature is about 180-280° C. and typically a copper/zinccatalyst supported on alumina (Cu/Zn/Al) catalyst is used. In theselatter stages the residual CO concentration in the outlet stream istypically as low as 0.1-0.3%. In case of a CPO process as the source ofthe syngas, also steam may be added in the WGS reactor. If needed, thehydrogen-depleted gas stream is transferred through a heat exchangerbefore entering the WGS reactor, to better control the inlet temperatureof the gas stream before the water gas shift reaction.

The gas stream resulting from the WGSR contains mainly hydrogen andcarbon dioxide. Optionally, hydrogen is separated from this stream bypressure swing absorption (PSA) to yield a pure hydrogen stream.Preferably, the PSA unit in the present invention is suitable forhandling gas streams having a low hydrogen content.

In a further interesting embodiment, an inert sweep gas, e.g. steam or,preferably, nitrogen, is applied downstream of the membrane. Asmentioned above, this has a beneficial effect in enhancing thetrans-membrane flux of hydrogen. A further advantage of this embodiment,is that the sweep gas can be applied to cool the membrane. By virtue ofthe relatively low thickness of the membranes used in the invention,viz. of from 1 μm to 3 μm, the sweep gas can be applied as the solecoolant for the membrane reactor. Thus, the use of sweep gas has asynergistic effect of enhancing hydrogen flux (leading to a moreefficient process), as well as the possibility to use it as a means tocontrol the temperature of the membrane. E,g., it can be used as acoolant, and then it is possible to dispense with additional coolingmeans (which has clear advantages in terms of energy consumption andprocess economics).

In another aspect, the present invention relates to a method of hydrogenand carbon dioxide production. In addition to the steps described above,the method has a further step of carbon dioxide separation after the WGSreaction. The CO₂ separation is performed using conventional means,e.g., chemical absorption techniques. In this way at least 90% andpreferably 99% of CO₂ is removed from the stream, which then can bedelivered as a product at battery limit. The remaining gas canadvantageously be used together with fuel gases for combustion. Theseparation of CO₂ from the stream improves further the thermalefficiency of the hydrogen production process.

In a preferred embodiment, before the CO₂ is removed from the gas streamoriginating from the shift reactor, an additional separation step ofhydrogen from the stream originating from the shift reactor isperformed. In this way the hydrogen production is further increased.This stream of hydrogen can be combined with the hydrogen streamoriginating from the first separation step, performed before the WGSreaction. Since in this case both hydrogen and carbon dioxide areremoved from the gas stream, no PSA is needed at the end of the process.A small PSA unit, however, may be used to further purify separatedhydrogen to obtain ultrapure hydrogen. Smaller PSA unit needed to purifyhydrogen produced is another advantage of the present invention.

As already mentioned, the process according to the present inventionachieves an improved conversion of CO into CO₂ and H₂ in the water gasshift reaction and, as a result, a higher overall hydrogen production.The process of the invention allows to achieve a better CO conversion atthe same reactor outlet temperature. In this way, the hydrogenproduction can be increased by 10-15% and the overall energy efficiencyof the system by 5-7%.

Additionally, a benefit of the present invention is that the COconversion and H₂ separation reactions can be decoupled and, therefore,the reaction conditions can independently be optimized. This has as anadvantage that a simpler membrane separator may be used for hydrogenseparation, which does not require catalyst to be embedded. In this waythe interference of the metals in the separation membrane and in thecatalyst of CO conversion is avoided. Further, the present inventionmakes it possible to easily achieve a higher hydrogen production and ahigher conversion by simply adding a hydrogen separation unit to analready existing plant. Therefore, minimal alterations to the existingequipment are required. The existing unit can also serve as a bypassaround the separation unit to allow for example maintenance in case ofupsets.

In another aspect, the invention relates to a method for increasing thecapacity of a hydrogen production plant, the method comprising adding amembrane separation unit for hydrogen separation between a syngasproduction reactor and a water gas shift reactor. For hydrogen plantswherein syngas is produced by steam reforming, a membrane separationunit is added between the steam reformer and the water gas shiftreactor. In a preferred embodiment, at least 70-90 vol. % of hydrogen isseparated in the additional step. In a further preferred embodiment, athin palladium-based membrane having a thickness of 1-3 μm is used forthe hydrogen separation in this additional step. The increase incapacity means increased production of hydrogen. This has as anadvantage that existing plants can be modified according to the methodof the invention to produce more hydrogen without considerable changesto the equipment. As illustrated in the examples, the capacity of anexisting hydrogen plant can be increased by 10%, while the energyrequirement per unit (caloric value kcal/Nm³) of the hydrogen producedis decreased by about 7%.

In yet a further aspect, the invention relates to a plant for hydrogenproduction suitable for the method according to the invention,comprising a reactor for the production of syngas from hydrocarbons,wherein an outlet of the reactor is connected to an inlet of a firstseparation module for hydrogen separation having an outlet for hydrogenand an outlet for a hydrogen-depleted stream, wherein the outlet for thehydrogen-depleted stream is connected to an inlet of a shift reactor forwater gas shift reaction. An outlet of the shift reactor is preferablyconnected to a PSA unit for hydrogen separation.

In a preferred embodiment, the plant is suitable for hydrogen and carbondioxide production and comprises a shift reactor provided with an outletconnected to a CO₂ removal unit, provided with an outlet for carbondioxide and an outlet for off-gases. Since in this case both hydrogenand carbon dioxide are removed from the gas stream, no PSA is needed atthe end of the process. A small PSA unit, however, may be used tofurther purify separated hydrogen to obtain ultrapure hydrogen. The PSAunit in this embodiment may be significantly smaller than in theembodiments in the prior art because the stream entering the PSA issmaller because part of the hydrogen has already been removed by theseparation step. However, when the H₂ separated before the WGSR is alsofed to the PSA, a conventional sized PSA unit may be used.

In a preferred embodiment, the plant is provided with a secondseparation module for hydrogen separation placed between the shiftreactor and the CO₂ removal unit, said separation module provided withan outlet for hydrogen and an outlet routed to the CO₂ removal unit. Ina further preferred embodiment, the reactor suitable for the productionof syngas is a steam reforming reactor, an autothermal reactor or acatalytic partial oxidation reactor. In another preferred embodiment,the plant is further provided with a heat exchanger placed between thefirst separation module and the shift reactor, to adjust the temperatureof the incoming gas stream.

The present invention will further be described with respect toparticular embodiments and with reference to certain drawings but theinvention is not limited thereto but only by the claims. Any referencesigns in the claims shall not be construed as limiting the scope. Thedrawings described are only schematic and are non-limiting. In thedrawings, the size of some of the elements may be exaggerated and notdrawn on scale for illustrative purposes. Where the term “comprising” isused in the present description and claims, it does not exclude otherelements or steps. Where an indefinite or definite article is used whenreferring to a singular noun, e.g., “a” or “an”, “the”, this includes aplural of that noun unless something else is specifically stated. Thepercentages are by volume unless indicated otherwise.

A typical known process for hydrogen production by steam reforming isshown in FIG. 1. A feed comprising hydrocarbons, e.g., natural gas ormethane, is first desulfurised in a hydrodesulfurisation (HDS) reactor111. A part of the feed can also be used as fuel to heat the steamreformer. The desulfurised feed is further supplied to the steamreformer (SR) 110, wherein the hydrocarbons are reacted with steam at ahigh temperature (such as 700-900° C.). Syngas obtained in the result ofthe steam reforming reaction is then passed to a process gas boiler(PGB) 100, wherein the syngas is cooled to the temperature of thesucceeding water gas shift (WGS) reaction. The cooling is usually donewith water so that the thereby formed steam can be used in the steamreforming reaction. The cooled syngas 10 is then transferred to a shiftreactor 103 wherein the WGS reaction takes place. The resulting stream13 comprising hydrogen and carbon dioxide is routed to a pressure swingabsorption (PSA) unit 104, wherein separation of hydrogen takes place.In an alternative embodiment a membrane separation unit can be usedinstead of a PSA. The hydrogen 14 can then be delivered at batterylimit, while remaining purge gas 15 can be used for combustion to heatup the reformer.

A preferred embodiment for the hydrogen production according to thepresent invention is shown in FIG. 2. Similarly to a conventional methoddescribed in FIG. 1, syngas is obtained first in a SR 110 from a feeddesulfurised in a HDS 111, and then cooled in a PGB 100. Subsequently,according to the invention, the cooled gas 10 is transferred to aseparation module 101 for hydrogen separation. In this separation modulefrom 70 to 90 vol. % of hydrogen is removed from the gas stream.Resulting stream 12 is a pure hydrogen stream (permeate) at lowerpressure (LP), in the range of 2-10 barg, which can be delivered atbattery limit. The hydrogen can also be compressed in a compressor 106and delivered at battery limit at a desired pressure, or delivered toPSA unit 104, for further purification in order to obtain ultrapurehydrogen. The process gas 11, which is depleted in hydrogen, passes aheat exchanger 102 wherein it is cooled down (if desired) to the workingtemperature of the shift reactor 103. Stream 13 resulting from the WGSR103 is cooled down to separate the condensate and is further transferredto a PSA unit 104 for the final purification of the hydrogen. Thehydrogen separated in the PSA 104 can be mixed with hydrogen stream 101,while remaining purge gas can be used to heat up the SR.

A preferred embodiment for both hydrogen and carbon dioxide productionaccording to the present invention is shown in FIG. 3. This scheme issimilar to that in FIG. 2, including a hydrogen separation module 101Adownstream to a PGB 100, a heat exchanger 102 and a shift reactor 103.Further in this scheme, the stream 13 at the exit of the shift reactor103 may be processed in an additional separation module 101B to separatehydrogen, or may be fed directly to a CO₂ removal unit 105. In the firstcase, the hydrogen-depleted stream 17 obtained from the separationmodule 101B is fed to a CO₂ removal unit 105, wherein the stream isseparated into a pure CO₂ product stream 19 and an off-gas 15 containingH₂, CO and CH₄, the latter preferably being used together with fuel gasfor combustion. In this case no PSA unit is required to produce the purehydrogen product. However, a small PSA unit 107 can be used at the exitof the hydrogen compressor 106, if it is desired to convert hydrogenstream 18 into an ultrapure hydrogen product 14. The remaining purge gas16 can be used together with fuel gases for combustion.

For the process schemes depicted in FIGS. 1 and 2, the composition ofdifferent gas streams is given in Tables 1 and 2, respectively. Thestreams are referenced by the same numbers as used in the processschemes. Stream 10 is a typical composition downstream to a SR.

TABLE 1 Material balance for the block diagram for producing H₂ viasteam reforming (SR) in FIG. 1. 10 13 14 15 Total Molar Comp. PercentsCH4 2.0 2.0 0.0 8.7 CO2 6.2 12.5 0.0 53.5 CO 8.9 2.6 0.0 11.0 H2 43.850.1 100.0 23.6 H2O 38.6 32.3 0.0 0.7 N2 0.5 0.5 0.0 2.5 FlowrateKG-MOL/HR 4386 4386 1954 1024 Temperature C. 350 418 38 38 PressureKG/CM2G 27.3 26.9 25.25 0.3

TABLE 2 Material balance for the block diagram for producing H₂ viasteam reforming (SR) in FIG. 2. 10 11 12 13 14 15 Total Molar Comp.Percents CH4 2.0 2.9 0.0 2.9 0.0 10.2 CO2 6.2 8.9 0.0 19.4 0.0 68.0 CO8.9 12.8 0.0 2.3 0.0 8.0 H2 43.8 18.9 100.0 29.4 100.0 10.3 H2O 38.655.6 0.0 45.1 0.0 0.5 N2 0.5 0.9 0.0 0.9 0.0 3.0 Flowrate KG-MOL/HR 43863043 1343 3043 2149 869 Temperature C. 350 350 350 456 111 38 PressureKG/CM2G 27.3 27.1 5.5 26.9 24.5 0.3

It is clear from Tables 1 and 2 that the flow of hydrogen in stream 14is increased by approximately 10%. Due to the fact that the gas stream13 fed to the PSA unit has a much lower CO content, the PSA efficiencyis improved and, as a result, the overall H₂ recovery factor is improvedby 0.5-1.5%. To compensate the reduced amount of purge gas 15 and itslower heating value, an additional small amount of natural gas is burntinto the SR to provide the required reforming duty. Although anadditional small amount of natural gas needs to be burnt, the overallsystem efficiency is higher since less feed is needed to produce sameamount of hydrogen.

In case of the process according to FIG. 2, the hydrogen production isincreased by 10%, while the caloric value kcal/Nm³ of produced hydrogenis decreased by about 7%. In the scheme of FIG. 2, hydrogen is producedat a relatively low pressure directly from the separation module andneeds to be compressed. However, even considering the additional energyneeded to compress hydrogen, the overall efficiency is still better thanin the conventional process.

Table 3 describes the stream composition for the embodiment illustratedin FIG. 3. The streams are referenced by the same numbers as used in theprocess scheme. The same composition is used for the initial feed ofsyngas 10, being a typical composition downstream to a SR.

TABLE 3 Material balance for the block diagram for producing H₂ and CO₂via steam reforming (SR) in FIG. 3. 10 11 12 13 14 15 16 17 18 19 TotalMolar Comp. Percents CH4 2.0 3.1 0.0 3.1 0.0 10.3 0.0 6.1 0.0 0.0 CO26.2 9.7 0.0 21.4 0.0 0.7 0.0 41.4 0.0 100.0 CO 8.9 13.8 0.0 2.1 0.0 6.80.0 4.0 0.0 0.0 H2 43.8 12.3 100.0 24.1 100.0 78.8 0.0 46.5 100.0 0.0H2O 38.6 60.2 0.0 48.4 0.0 0.4 0.0 0.2 0.0 0.0 N2 0.5 0.9 0.0 0.9 0.03.0 0.0 1.8 0.0 0.0 Flowrate KG-MOL/HR 4386 2815 1571 2815 1571 860 0.01457 1571 597 Temperature C. 350 350 350 466 38 38 38 38 155 38 PressureKG/CM2G 27.3 27.1 5.5 26.9 23.8 25.3 0.3 26.0 24.5 0.3

In the process scheme of FIG. 3 hydrogen is further purified in asmaller PSA unit, while the gas after the CO₂ removal unit is routeddirectly to the SR and used as supporting fuel if its caloric value ishigh enough. The removal of CO₂ from the process stream further improvesthe thermal efficiency of the SR reformer.

1. A method for hydrogen production, comprising the steps of: (i)providing a gas stream comprising hydrogen and carbon monoxide; (ii)separating at least 70 vol. % of hydrogen from the stream by membraneseparation using a thin palladium-based membrane having a thickness of1-3 μm, yielding a hydrogen-depleted stream; (iii) subjecting thehydrogen-depleted stream to a water-gas shift reaction; and, optionally,(iv) separating hydrogen from the stream resulting from step (iii). 2.The method of claim 1, further comprising separating carbon dioxide fromthe stream resulting from step (iii).
 3. The method according to claim1, wherein the stream of step (i) contains syngas originating from asteam reformer, autothermal reactor and/or catalytic partial oxidationreactor.
 4. The method according to claim 1, wherein in step (ii) 70-90vol. % of hydrogen is separated.
 5. The method according to claim 1,wherein the separation of hydrogen in step (ii) is performed using athin palladium-based membrane, working at 400-450° C.
 6. The methodaccording to claim 2, which further comprises after step (iii) andbefore the separation of CO₂, separating hydrogen from the gas streamresulting from step (iii).
 7. The method according to claim 1, whereinat least part of the separated hydrogen is purified by pressure swingabsorption.
 8. A method for increasing the capacity of a hydrogenproduction plant, the method comprising adding a membrane separationunit for hydrogen separation between a syngas production reactor and awater gas shift reactor.
 9. The method according to claim 6, wherein atleast 70-90 vol. % of hydrogen is separated in said separating.
 10. Themethod according to claim 6, wherein said separating is performed usinga thin palladium-based membrane having a thickness of 1-3 μm.
 11. Themethod according to claim 1, wherein an inert sweep gas is applieddownstream of the membrane.
 12. A plant for hydrogen productionaccording to the method of claim 1, comprising a reactor 110 for theproduction of syngas from hydrocarbons, wherein an outlet of the reactor110 is connected to an inlet of a first separation module 101, 101A forhydrogen separation having an outlet for hydrogen and an outlet for ahydrogen-depleted stream, wherein the outlet for the hydrogen-depletedstream is connected to an inlet of a shift reactor 103 for water gasshift reaction.
 13. The plant according to claim 12, wherein an outletof the shift reactor 103 is connected to a pressure swing absorptionunit 104 for hydrogen separation.
 14. The plant according to claim 12,wherein an outlet of the shift reactor 103 is connected to a CO₂ removalunit 105, provided with an outlet for carbon dioxide and an outlet foroff-gases.
 15. The plant according to claim 14, further provided with asecond separation module 101B for hydrogen separation placed between theshift reactor 103 and the CO₂ removal unit 105, said separation module101B provided with an outlet for hydrogen and an outlet routed to theCO₂ removal unit
 105. 16. The plant according to claim 12, wherein thereactor 110 for the production of syngas is a steam reforming reactor,an autothermal reactor or a catalytic partial oxidation reactor.
 17. Theplant according to claim 12, further provided with a heat exchanger 102,placed between the first separation module 101 and the shift reactor103.
 18. The method of claim 11 wherein the sweep gas is nitrogen.